Process for purifying natural gas

Abstract

Processes are disclosed for separating water and carbon dioxide from hydrocarbon streams, e.g., natural gas, by adsorption prior to low temperature hydrocarbon processing operations, e.g., liquid natural gas recovery and nitrogen rejection. The processes employ separate adsorption zones for the water and carbon dioxide adsorption and both adsorption zones are operated with a thermal swing adsorption cycle. A product gas from the low temperature processing operation, e.g., methane, can be used as the purge gas for one or both adsorption zones and the desorbed water and carbon dioxide, along with the purge gas can be recovered and introduced into a natural gas pipeline, for example.

We claim: 
     
       1. A process for separating water and carbon dioxide from a feedstream comprising hydrocarbons, water and carbon dioxide, said process comprising the steps of: (a) passing the feedstream to a first adsorption zone containing a solid adsorbent having selectivity for water at a first zone adsorption temperature effective to adsorb water and a first zone adsorption pressure and withdrawing a first zone adsorption effluent stream having a reduced concentration of water relative to the feedstream;   (b) passing at least a portion of the first zone adsorption effluent stream to a second adsorption zone containing a solid adsorbent having selectivity for carbon dioxide at a second zone adsorption temperature effective to adsorb carbon dioxide and a second zone adsorption pressure, said second zone adsorption temperature being lower than said first zone adsorption temperature, and withdrawing a second zone adsorption effluent stream having a reduced concentration of carbon dioxide relative to the first effluent stream;   (c) passing a first purge gas through the first adsorption zone at a first zone regeneration temperature effective to desorb water from the first adsorption zone and a first zone regeneration pressure and withdrawing a first zone desorption effluent stream comprising water; and   (d) passing a second purge gas through the second adsorption zone at a second zone regeneration temperature of at least 250° F. and effective to desorb carbon dioxide from the second adsorption zone and a second zone regeneration pressure, said second zone regeneration pressure being not lower than about 50 psia, and withdrawing a second zone desorption effluent stream comprising carbon dioxide.   
     
     
       2. The process of claim 1 wherein the first zone regeneration temperature is higher than the second zone regeneration temperature. 
     
     
       3. The process of claim 1 wherein the feedstream comprises methane and at least one other hydrocarbon selected from ethane, propane and butane. 
     
     
       4. The process of claim 3 further comprising separating the second zone adsorption effluent into a first fraction comprising methane and a second fraction rich in said at least one other hydrocarbon. 
     
     
       5. The process of claim 4 further comprising reducing the temperature and pressure of the second zone adsorption effluent by expansion prior to separating said second zone adsorption effluent to provide an expanded adsorption effluent substantially free of hydrates. 
     
     
       6. The process of claim 4 wherein either one or both of the first purge gas and the second purge gas comprise at least a portion of the first fraction. 
     
     
       7. The process of claim 4 further comprising passing a first portion of the first fraction to the second adsorption zone to comprise at least a portion of the second purge gas and passing at least a portion of the second zone desorption effluent to the first adsorption zone to comprise at least a portion of the first purge gas. 
     
     
       8. The process of claim 7 further comprising combining at least a portion of the first zone desorption effluent with a second portion of the first fraction to provide a combined product gas stream comprising methane. 
     
     
       9. The process of claim 4 wherein the feedstream further comprises at least one light gas selected from nitrogen, helium, argon, oxygen and hydrogen. 
     
     
       10. The process of claim 4 wherein the first fraction comprises said at least one light gas. 
     
     
       11. The process of claim 10 further comprising separating the first fraction into a rejection stream comprising said at least one light gas and a raw product gas stream comprising methane. 
     
     
       12. The process of claim 11 comprising diverting a portion of the raw product gas to the second adsorption zone to comprise at least a portion of the second purge gas and passing at least a portion of the second zone desorption effluent to the first adsorption zone to comprise at least a portion of the first purge gas. 
     
     
       13. The process of claim 12 comprising cooling the first zone desorption effluent to a temperature sufficient to condense water therefrom and combining the water-depleted effluent with the undiverted portion of the raw product gas to provide a combined product stream. 
     
     
       14. The process of claim 1 wherein the first zone adsorption temperature is from about 30° to 150° F. 
     
     
       15. The process of claim 1 wherein the second zone adsorption temperature is from about -40° to 120° F. 
     
     
       16. The process of claim 1 wherein the first zone regeneration temperature is from about 400° to 650° F. 
     
     
       17. The process of claim 1 wherein the second zone regeneration temperature is from about 300° to 550° F. 
     
     
       18. The process of claim 1 wherein the first zone adsorption pressure is from about 100 to 1700 psia and the first zone regeneration pressure is from about 50 to 1500 psia. 
     
     
       19. The pressure of claim 1 wherein the second zone adsorption pressure is from about 100 to 1700 psia and the second zone regeneration pressure is from about 50 to 1500 psia. 
     
     
       20. The process of claim 1 further comprising diverting a portion of the first zone adsorption effluent around the second adsorption zone and combining the diverted portion with the second zone adsorption effluent to provide a combined adsorption effluent. 
     
     
       21. The process of claim 20 wherein said portion is sufficient to provide a concentration of from about 1 to 250 ppmv of carbon dioxide in the combined adsorption effluent. 
     
     
       22. A process for separating water and carbon dioxide from a natural gas feedstream comprising said water, said carbon dioxide, methane, at least one other hydrocarbon selected from ethane, propane, butanes, pentanes and hexanes, and at least one light gas selected from nitrogen, helium, argon, oxygen and hydrogen, said process comprising: (a) passing the feedstream to a first adsorption zone containing a solid adsorbent having selectivity for water at a first zone adsorption temperature effective to adsorb water and a first zone adsorption pressure and withdrawing a first zone adsorption effluent stream having a reduced concentration of water relative to the feedstream;   (b) passing at least a portion of the first zone adsorption effluent stream to a second adsorption zone containing a solid adsorbent having selectivity for carbon dioxide at a second zone adsorption temperature effective to absorb carbon dioxide and a second zone adsorption pressure, said second zone adsorption temperature being lower than said first zone adsorption temperature, and withdrawing a second zone adsorption effluent stream having a reduced concentration of carbon dioxide relative to the first effluent stream;   (c) separating at least a portion of the second effluent stream into a raw product gas stream comprising methane and a rejection stream comprising said at least one light gas;   (d) passing a first purge gas comprising a portion of the raw product gas to the first adsorption zone at a first zone regeneration temperature effective to desorb water from the first adsorption zone and a first zone regeneration pressure and withdrawing a first zone desorption effluent stream comprising water;   (e) passing a second purge gas comprising a portion of the raw product gas to the second adsorption zone at a second zone regeneration temperature of at least 250° F. and effective to desorb carbon dioxide from the second adsorption zone and a second zone regeneration pressure, said second zone regeneration pressure being not lower than about 50 psia, and withdrawing a second zone desorption effluent stream comprising carbon dioxide; and   (f) combining the first zone desorption effluent with the remaining portion of the raw product gas to provide a combined product gas rich in methane.   
     
     
       23. The process of claim 22 wherein the first purge gas comprises the second zone desorption effluent stream. 
     
     
       24. The process of claim 23 comprising heating the second purge gas to the second zone regeneration temperature prior to said passing to the second adsorption zone and further heating the second zone desorption effluent stream to the first zone regeneration temperature prior to said passing to the first adsorption zone, said first zone regeneration temperature being higher than said second zone regeneration temperature. 
     
     
       25. The process of claim 22 comprising cooling the first zone adsorption effluent prior to said passing to the second adsorption zone. 
     
     
       26. The process of claim 22 wherein the second zone adsorption effluent is separated in a first distillation column to provide a demethanizer overhead stream comprising methane and a demethanizer bottoms stream comprising said at least one other hydrocarbon and the demethanized overhead stream is separated in a second distillation column to provide the rejection stream and the raw product gas.